The invention relates to a method and means for preparing an elongated object having a relatively high bending stiffness, such as an elongated tube or cable, before and while sorting and/or transporting said object.
For laying pipelines on the sea bed one could use the option whereby short pipe sections are transported to a special pipe laying vessel, which travels along the traject where the pipe has be laid. On that vessel the pipe segments are welded together and the resulting long pipeline is lowered into the sea. This method and the vessels to perform this method are widely known. The operation of these specialized vessel is rather slow so that the chance of meeting bad weather, rough seas and other non favourable conditions is not imaginary.
Another option is to weld the pipe segments together into a pipeline on the shore, to drag the line into the water after providing sufficient buoyancy to the line to keep the line afloat and to use one or more tugboats to transport the floating line to its destination where a specially equipped pipe laying vessel is used for lowering the pipeline onto the seabed. Although the pipe laying vessel could operate significant faster in this case because all the welding is already done, this option is only feasible for restricted length of pipelines, e.g. a few hundred meters. Longer pipelines result into serious navigational problems and pipe line lengths of e.g. 100 km are definitely out of the question. Examples of methods which fall into this category are described in U.S. Pat. No. 4,798,500 and U.S. Pat. No. 3,698,348.
For transporting long pipelines one has already proposed to use large reels onto which the pipeline is wound. The reel with the thereon wound pipeline should have sufficient buoyancy to keep the combination afloat. Indeed relatively long pipelines having a length of many km can be transported in this way using one or more tugboats to the position where the pipe laying vessel is waiting to lower the pipeline onto the sea bottom. A prior art example of this option is found in U.S. Pat. No. 4,117,692.
Instead of floating reels it is also known to use special pipe laying vessels which are able to transport the reel whereby the reel is supported on a horizontal axis. An example is described in U.S. Pat. No. 4,802,794. At its destination the reel is brought into a rotating motion around said axis and the pipeline is unwound and lowered down.
A serious disadvantage is formed by the reel itself. Reels of large dimensions are very expensive especially if they have to carry large weights in the order of for instance 35,000 tons or even more as in the case of long pipelines. Even if such relatively large reels are used still they are only practicable for tubes with a restricted cross-section and a relatively large flexibility. If a reel is used with a diameter of e.g. 10 m and a width of 4 m for storing a pipeline width a diameter of 20 cm then still a monolayer winding will only hold approximately 600 m length of pipeline. In more layers it would be possible to store a pipeline with a length of a few km on such a reel.
Such loaded reels become hard to handle especially installed on a horizontal shaft onboard of a ship, on the one hand because of the dimensions and on the other hand because of the weight thereof. If longer lines are wound in more than one layer on a reel a further disadvantage is the transition between the various layers. Deformation of the tube at said transition is hardly avoidable.
The object of the invention is now to provide a method for sorting and/or transporting a pipeline, which method can be used with very long pipelines of relatively large diameter.
In agreement with said object the invention provides a method for preparing an elongated object having a relatively high bending stiffness, such as an elongated tube or cable, before storing and/or transporting said object, which method is characterised in that
a) the elongated object is provided with sufficient buoyancy capacity to enable the object to float on a liquid with a predetermined specific gravity and the
b) the elongated object, while floating on the liquid and carried thereby, is spirally wound into a flat monolayer spiral, the inner radius of which is preferably in the order of the diameter of the elongated body divided by the yield point (ratio) or larger of the most fracture critical material in the elongated object (e.g. concrete liner or jacket).
The method according to the invention does not use a reel. The pipeline itself has its own buoyancy and floats on the liquid. It might be surprising but even large diameter tubes which are rather stiff (e.g. tubes with a concrete jacket to provide sufficient weight to maintain the pipe on the sea bottom) can be bend into a circle with a diameter of a few hundred meters without damaging the tube. The bending operation can be performed with relatively small bending forces and with relatively simple means as will be discussed later.
As soon as the pipeline is prepared for transport in this manner it is possible to test the quality of the pipeline by closing both ends of, pressurizing the closed tube and inspecting the tube with apparatuses known as such. If the pipeline fulfills the requirements the transporting operation can be started.
To avoid that the pipe sections of which the pipeline is assembled have to be welded together in a wet environment while the segments are floating it is preferred that before performing step a) or during the performance of step a) the elongated body is assembled from smaller sections on a solid surface. This solid surface may be a land surface near the shore of a lake, sea, ocean etc. The pipe is welded together on the shore, buoyancy elements are attached and the pipe is drawn into the lake or sea. In stead thereof said solid surface may be the deck of a ship, pontoon or other floating vessel where the pipe is welded together on deck, buoyancy elements are attached and the pipe is drawn into the lake or sea. It is rather obvious that in the abovementioned cases the liquid will be (sea)water. However, also other liquids can be used.
It will be clear from this embodiment that application of the invention is not confined to large stretches of water, but can be brought into practice on land as well. The advantage with reference to the pending situation is that handling a floating spirally shaped tube is for more easier than disassembling and assembling a whole pipe string each time for instance the drill bits have to be replaced.
Although the gutter may be filled with water in the direct neighbourhood of a drilling rig site there are large volumes of drilling fluid which have to be stored in reservoirs, tanks etc. Within the scope of the invention it is now proposed to store the drilling fluid in a gutter so that the liquid, onto which the pipeline floats, consists of drilling fluid.
The elongated object can be provided with at least a part of the necessary buoyancy by closing both ends of the pipeline and filling the pipeline with a substance of which the specific gravity is lower than the specific gravity of (sea)water. This substance could be a gas, preferably air. An advantage of this option is that a simple pressure meter measuring the pressure inside the pipeline is sufficient to provide an indication of the air-tightness of the pipeline during storage and transport thereof.
Furthermore, the elongated object can be provided with sufficient buoyancy capacity by attaching a number of buoyancy bodies to the spirally wound pipeline, preferably in an approximately equiangular configuration, or by attaching a coating of a material with low specific weight to the outside wall of the pipeline.
To create and maintain the shape of the spiral configuration of the pipeline during winding, during transport and during unwinding and to prevent therewith a to strong deformation from the desired shape a guiding means is used which comprises a body with a bow shaped side wall, the shape of which corresponds with the desired shape of the inner winding of the spirally wound pipeline.
During winding and unwinding operations preferably measures are taken to assure a low friction movement of the pipeline along said side wall. However, during transport said body is attached to the inner winding of the spiral such that the centre of said body is positioned on a line through the center of the spiral in the direction of transport or in the prevailing direction of the combined forces exerted by wind and/or water flows.
It is remarked that a tube wounded into a flat monolayer spiral is used also in WO 97/41054. However, in that case the spiral has no buoyancy and is carried by waggons running on a circular stretch of railway. From the wagons the tube is transferred to the installations on board a pipe laying vessel.
Further detail of the method as well as details of the means which are preferably used when the method is performed, are provided in the following specification part with reference to the accompanying drawings.